1. Field of the Invention
This invention relates to casters and, more particularly, to caster assemblies that allow the caster to pivot easily to accommodate the direction of thrust applied to an object supported by the caster.
2. Description of Related Art
Caster designs tend to fall into one of two groups: the ‘single wheel’ and the ‘twin wheel’. The single wheel caster consists of one wheel with an axle through the center which is attached to a clevis-like bracket that supports the axle at opposed ends. The bracket is joined to a bearing assembly that allows the caster to pivot about a generally vertical axis while also permitting rotation of the wheel about a generally horizontal axis, thereby allowing the caster to roll in any direction. The bracket is offset from vertical by a head angle, and the bracket may include a rake offset, so that the caster bracket will tend to rotate about the pivot axis and extend in a direction that is directly opposed to the direction of motion of the object supported by the caster. This arrangement, known in the
The main problem of the ‘single wheel’ caster is its inability to turn easily about the pivot. When changing direction, the wheel is required to rotate about the vertical axis of the bracket bearing assembly. Differing portions of the contact area of the wheel describe arcs of differing radii, requiring differing linear velocities, thus necessitating that some of the contact area is in rolling contact while other portions are sliding and dragging (“scrubbing”). The result is wear of the wheel and greater rolling resistance at low speeds and tight turns, and most particularly high rolling resistance during startup from zero velocity. A narrower wheel minimizes this problem, but results in greater pressure (force per unit area) applied to the floor surface.
The ‘twin wheel’ caster offered an improvement over the ‘single wheel’ in two important regards. The ability of the wheels to rotate at differing rates or in opposite directions at the same time greatly enhances the ability to turn about the vertical pivot axis, making a change in overall direction of the object much smoother. Also, the separation of the two wheels establishes a wide base for stability. One problem germane to this caster type is the method of attachment of the wheel to the axle. Unlike the single wheel with the clevis bracket supporting the outer ends of the axle, the double wheel design typically attaches each wheel to its respective end of the axle, the vertical pivot shaft being disposed in a plane between the two wheels. The wheels and the ends of the shaft are relatively unprotected and vulnerable to impact, which can result in collision-damaged wheels or a bent axle. This attachment configuration also creates some inherent stability inefficiencies. By not allowing the axle to extend through the wheel, the loading of the wheel on the axle is not symmetric. That is, the inside of the wheel is fully loading the axle while the outside is not loading the axle at all. Also, the limited space remaining in the device for the wheel thickness results in thin wheels, which directly result in narrow annular (limited) bearing surfaces of the wheel on the axle. The twin wheel design is typically limited to use in office furniture, where the loads are lighter and the objects are not often moved.
A problem that is common to single wheel and twin wheel casters is that the casters can become “locked up” when at least two casters supporting a wheeled object are pivoted to extend in directions that are substantially misaligned with each other. For example, if a cart with four swiveling (pivoting) casters is pushed toward a wall and abutted against the wall surface, it may then become difficult to slide the cart along the wall to reposition it, due to the fact that the casters are misaligned with respect to the plane of the wall surface. In general, when casters, either single wheel or twin wheel, are forced to pivot about the contact area that they engage on a floor or carpet, their rotational movement creates a substantial frictional resistance due to the fact that the wheels are not primarily rolling but rather are “scrubbing” on the contact area. The result is that objects supported by casters may be difficult to start to roll in a desired direction if that direction does not align with the casters. And precision placement of the object may involve a great deal more maneuvering than would otherwise be necessary if the casters were capable of easily tracking the direction of thrust.